feat: ✨ add support for reading credentials from the environment variables

[zoido]

• extend Discovery class with new field

• implement loading by the agreed protocol

• include loading of the credentials in the discovery functions

• Protocol was agreed in https://github.com/h2oai/cloud-discovery/issues/318#issuecomment-1552538282

• API shape was established as https://github.com/h2oai/cloud-discovery/issues/317#issuecomment-1559149902

RESOLVES https://github.com/h2oai/cloud-discovery/issues/436